1. Field of the Invention
The invention relates to a polarization converter for an unpolarized light, and more particularly to a polarizing beam splitter (BSP) used in a liquid crystal display system.
2. Description of the Related Art
In a conventional polarizing plate in a liquid crystal projection system, energy loss is caused during light absorption. To develop a high efficient polarizing device is therefore a topic studied intensively in industry.
In FIG. 1, an improved conventional polarizing system is shown. A light source 10 is reflected and collimated as a light beam 14 by a parabolic mirror 12. The light beam 14 is incident on a PSB 16 with an angle of 45.degree.. After the PSB 16, the parallel light beam 14 is split into a parallel (P-) polarized light 14a and a senkrecht (S-) polarized light 14b. The P-polarized travels forward in the same direction as the incident light beam 14. The S-polarized light 14b travels with a right angle of the light beam 14. The S-polarized light 14b is further incident and reflected by a reflecting mirror 18 with an angle of 45.degree.. The S-polarized light 14b travels further and is incident on a half wave plate 20. After the half wave plate 20, the S-polarized light is then converted into a P-polarized light 14c which travels parallel to the P-polarized light 14a. In the above system, the light is polarized and split. However, as shown in the figure, the cross section of the light beam is multiplied, and it is more difficult for light distributing and converging.
A technique which modifies the above PBS into an array is disclosed in "SID 97 DIGEST, pp.993-996, 1997" by Itol et al. The structure of the optical layout can be referred to FIG. 2a and FIG. 2b. In FIG. 2a and FIG. 2b, a light source 20 is reflected as a collimated light beam 24 by a parabolic reflector 22. While traveling through a first lens array 26, that is, a light distributor lens array, and a second lens array 28, that is, a light integrator lens array, the collimated light beam 24 is converged on a prism in the PBS 30. After the prism, the collimated light beam 24 is split into a P-polarized light P1 traveling straight forward and a S-polarized light S1 deflected with a right of about 90.degree.. The S-polarized light S1 is reflected by a reflecting mirror, and then travels parallel to the P-polarized light P1. Through a half-wave plate, the P-polarized P1 is converted as an S-polarized light S2. The S-polarized lights S1 and S2 are converged by a light condenser lens 32. The converged light is then projected on a liquid crystal light valve (LCLV) 34. By the lens arrays and the PBS 30 in a form of an array, a light source is split and polarized. The dimension of reflecting and polarizing mirrors are compact. The cross section of the resultant light is consequently reduced to achieve light distributing and polarizing. However, the fabrication is difficult, the fabrication cost is high, and the efficiency is degraded by the array combination.
An equivalent Wollaston prism with a small size and low cost is disclosed in US Pat. No. 5,448,407. Two prisms are assembled to polarize and split an incident light. In FIG. 3, an equivalent Wollaston prism comprises a first prism 62 and a second prism 64. The first prism 62 has an incident plane 622, a polarizing splitting plate 624, and an emerging plane 626. The second prism 64 has a boundary plane 644 and a reflecting plane 626. A beam of incident light L1 enters the equivalent Wollaston prism 62 through the incident plane 622, having its optical axis directed towards the polarizing splitting plate 624. The incident light L1 is split into one perpendicular P-polarized light LP emerging through the emerging mirror 626, and an S-polarized light LS traveling further and reflected by the boundary plane 644 with a right angle. The S-polarized light LS is then emerging through the emerging mirror 626 in a same direction of the P-polarized light LP. The incident light is thus split and polarized. In the equivalent Wollaston prism, the first prism 62 has functions of splitting, polarizing, and reflecting light, while the second prism 64 has the only function of reflecting light. The resultant emerging light comprises two light beams with different polarization. To obtain a resultant light comprises two light beams, other equipment or apparatus is required.